1. Field of the Invention
The present invention relates to a laser diode, an optical pickup device, an optical disk apparatus, and optical communications equipment, and more particularly to a laser diode which can reduce a radiation angle θ⊥ of a laser beam in the vertical direction, and an optical pickup device, an optical disk apparatus, and optical communications equipment employing a laser diode for their respective light-emitting units.
2. Description of the Related Art
In general, a laser diode used in an optical disk apparatus such as a CD (compact disk) and a DVD (digital versatile disk) or an optical pickup device built therein or optical communications equipment is of the stripe structure.
FIG. 10 shows a cross-sectional view of a conventional laser diode. For example, on an n-type substrate 10, there are laminated, for example, an n-type cladding layer 11 constituted by Al0.5Ga0.5As, and an active layer, which includes an upper layer of a quantum well structure composed of a well layer and a barrier layer and a lower layer of an SCH (Separated Confinement Hetero) optical guide layer, and which is, for example p-type first cladding layers 13, 15 constituted by Al0.5Ga0.5As, whereas a p-type optical guide layer 14 constituted by Al0.3Ga0.7As is formed on the border with the p-type first cladding layers 13, 15.
At a current injecting stripe region, the p-type first cladding layer 15 and the p-type optical guide layer 14 are processed in the shape of a ridge. At both sides of the current injecting stripe region, there is laminated on the p-type first cladding layer 13, for example, a n-type current block layer 16 constituted by Al0.5Ga0.5As.
On the p-type first cladding layer 15 and the n-type current block layer 16, there are laminated p-type second cladding layers 17, 18 of the embedded type constituted by Al0.55Ga0.45As, and, a p-type contact layer 19 constituted by GaAs is formed further on of these layers.
A p-electrode 20 is formed covering the p-type contact layer 19 to make an ohmic contact, and an n-electrode 21 is formed covering the bottom of the n-type substrate 10 to make an ohmic contact.
In the structure described above, the p-type optical guide layer 14 is a film that operates as an etching stopper as well, whereby, in the process of making the p-type optical guide layer 14 and the p-type first cladding layer 15 into a ridge shape, etching is once suspended on the surface of the p-type optical guide layer 14, so that the etching conditions are altered or the etching time is controlled to process up to and including the p-type optical guide layer 14, thus obtaining the ridge shape.
A laser diode of a conventional construction described above has an inherent constraint that transverse mode must be stabilized to restrict the generation of a kink. Stabilization is typically achieved by restricting a higher mode of the transverse mode, which calls for restraining the current field from expanding, thus necessitating to position the p-type optical guide layer 14 as close as possible to the active layer 12. For example, a distance between the p-type optical guide layer 14 and the active layer 12 is set on the order of 0.2 to 0.3 μm.
Nonetheless, in a conventional laser diode of the foregoing construction, there is a problem stemming from its structural constraint, that is, a radiant angle θ⊥ in the vertical direction of a laser beam growing larger than a radiant angle θ// in the horizontal direction.
The reason for the radiant angle θ⊥ in the vertical direction of laser beam becoming larger than the radiant angle θ// in the horizontal direction thereof is as follows: light is enclosed in the vertical direction related to the θ⊥ in a extremely thin region on the order of several hundred nm, which is a dimension of a thickness of the active layer, in contrast to light enclosed in the horizontal direction related to the θ// in a width of a few μm, which is a dimension of a stripe width, hence, when light is radiated as a laser beam, a strong diffraction in the vertical direction occurs in comparison with the horizontal direction.
As is apparent from the foregoing, positioning the p-type optical guide layer 14 in FIG. 10 closer to the active layer 12 to stabilize transverse mode causes light in the vertical direction to be enclosed more and more in the vicinity of the active layer and further causes the NFP (near field pattern) to become flatter and flatter in shape, while the FFP (far field pattern) which is its Fourier transform expands more and more in the vertical direction, thus enlarging the θ⊥.
The growth of θ⊥ in optical communications equipment mounted with a conventional laser diode produces a disadvantageous effect of a significant drop in coupling efficiency with transmission media such as an optical fiber as well as coupling efficiency with an optical system such as an optical pickup device. Consequently, when a conventional laser diode is used for an optical disk apparatus such as a CD-R/RW that requires a high-speed write operation, any component not conducive to optical coupling becomes a loss, so that the output requirement of the laser diode grows higher and higher.
Further, when it is used as a high-power laser for other purposes, too, it is preferable that the loss be decreased by improving optical coupling efficiency.
As conventional methods of reducing the θ⊥, there is known a method of decreasing a difference of refractive index between the active layer and the cladding layer or a method of making the film thickness of the active layer thin. However, these methods decrease a volume of the active layer, causing a gain to be insufficient, hence, an increase in a threshold current or an increase in an operating current may be induced.
Still further, Japanese Patent Application Publication (KOKAI) No. HEI 6-104525 contains a disclosure of another known method of regulating the radiating angle by varying a refractive index distribution in the vicinity of the active layer. Nevertheless, regulating the refractive index in the vicinity thereof means that a portion in which the refractive index is lowered by varying a composition ratio of Al acts as a barrier to the behavior of a carrier thus to make it difficult for a current to be injected into the active layer, thereby causing problems such as an increase in the operating current.
In this manner, in the currently available technology, difficulty of reducing the θ⊥ bring limits in flexibility of laser design.
The present invention is directed to resolving the current circumstances. Accordingly, the present invention provides a laser diode capable of reducing a radiant angle θ⊥ without generating any new problem described above, an optical pickup device, an optical disk apparatus, and optical communications equipment, all provided with a laser diode of the present invention to improve optical coupling efficiency.